1. Field of the Invention
The present invention relates to a technique for repairing defects formed in a photomask, and more particularly, to a repair method and apparatus for repairing defects such as projections (black defects) extending off a pattern.
2. Description of the Related Art
Conventionally, to repair a micro pattern formed in the photomask, a converged ion beam repair apparatus, which employs gallium as an ion source, has been used. Repair is performed by the apparatus in the following procedure:                (1) An appropriate region of a photomask around a defect is scanned with an ion beam and secondary electrons or secondary ions emitted from the photomask are detected to form an image;        (2) The size and position of the defect on the formed image and the positional relationship between the defect and a non-defective pattern are identified; and        (3) the defect is irradiated with a beam while an appropriate etching gas is supplied.        
In the step (1), an ion beam must be injected into the peripheral region of several μm around the defect to be repaired even though the peripheral region is not defective. If gallium ions are implanted in a light transmissible portion (glass portion) of the photomask, “gallium stain” is produced, which decreases light transmittance. In short, if gallium ions are implanted excessively, the transferred image on a wafer may sometimes be negatively affected. Therefore, irradiation must not be performed in excess of a standard dose.
It is known that the effect of the gallium stain increases as the wavelength of light emitted from light-emitting device decreases. More specifically, as the light-emitting device comes to employ an ArF laser (λ: 193 nm) in place of a KrF laser (λ: 248 nm), the permissible dose for obtaining an image decreases. Furthermore, when a laser of the next generation, F2 laser (λ: 157 nm), comes to be used, even a trace amount of gallium ions influences the image-transfer. It follows that such a gallium ion beam may not be used in practice.
If a defect is repaired but the repair is not made accurately in position, the defect must be re-repaired. However, because of the gallium-stain, the number of re-repair operations is limited. This problem takes place when any generation (type) of laser is used. In addition, in the step (3), since gallium ions are implanted into the underlying substrate of the defect, the effect of such gallium ions cannot be ignored. This phenomenon is caused by overetching.
To overcome these problems, it has been proposed that an electron beam should be used in place of an ion beam. However, the method of scanning a thin-converged electron beam lengthwise and crosswise, as is often performed in the ion optical system has a problem in that the etching rate is low compared to a method of scanning an excited ion beam. The low-etching rate problem can be overcome by increasing the beam size or the pixel size for beam scanning. However in this case, if the beam is applied along a edge of diagonal pattern, the edge of the pattern is processed in a zigzag fashion to form a stepped portion. When the beam is applied along a linear pattern, if the width of the pattern is not equal to an integral multiple of a pixel size, overetching or underetching occurs by at most a half-pixel-size.
The profiles are shown in FIGS. 8A to 8C. FIG. 8A shows that defects 83 and 84 are present between adjacent patterns 81 and 82. A rectangular beam 85 is scanned across the defects 83 and 84 to remove them, as shown in FIG. 8B. As a result, edge-roughness is produced as shown in FIG. 8C. The edge-roughness problem can be overcome by minimizing the beam size and a pixel size for beam scanning; however, the etching rate becomes low. The intrinsic problem to an electron beam is not overcome.
As described above, an electron beam may be used in place of an ion beam to remove defects (black defects) formed in a photomask. Since the etching rate of the electron beam is lower than that of the ion beam, extremely long time is required for repairing the defects. Conversely, if the size of electron beam is enlarged, and edge roughness is produced along a diagonal pattern (line).